In the modern society, people require higher to the life; baking, cooking, making tea or coffee are necessary in the daily life. Especially in the western-style baking, it has strict rules to the ingredients and the flow of water. User usually needs a measuring cup to get the water. It often happens that when you take the measuring cup under the tap to get the water, the measuring cup is filled up in one second when in high flow volume, it far exceeds the needed volume, which may waste the water. When in low flow volume, it gets slowly, during the process, user needs to take care of the volume in the cup to turn off the tap, it is a waste of time and labor. To solve the problems, a rationed tap exists in the market, this rationed tap can supply rationed flow volume according to the setting, the tap is turned off after the flow volume reaches to the preset value, so that the user needn't to take care of the volume. However, existing rationed tap is usually disposed with a flow adjusting portion, the user adjusts the proper flow volume and the water flows out. But misoperation may happen that the user adjusts the flow volume, but he doesn't get ready to get the water, water still flows out of the tap, it causes inaccurate of the water volume.